In a typical radio system (see FIG. 1), information is modulated onto a radio carrier by a transmitter. This signal then travels via an unknown and changing environment to the receiver. The ability to remove the effects of the environment from the signal is often key to the performance of a receiver.
The transmitter 101 passes information bits through a block adding error protection coding 102 and then through a modulation block 103 which modulates the coded information onto a radio carrier. As part of the modulation, known symbols may be added to assist with radio channel estimation in the receiver.
Once transmitted, the radio signal then passes through the radio channel 104 before reception 108. This radio channel frequently gives rise to intersymbol interference (ISI) which must then be removed by the receiver to ensure correct reception. Before being processed by the receiver blocks, the signal also acquires both interference and noise. The interference arises from other users of the spectrum whilst the noise is thermal noise from the environment. Additional noise is then added as the signal passes through the Rx front end 105.
The receiver 108 converts the analogue radio signal to a digital base band signal in the Rx front-end 105. The signal is then passed through the demodulation block 106. This serves to estimate the transmitted coded-bits in the presence of the ISI, interference and noise added by the radio channel and the Rx front end. The signal is then decoded 107 to yield the final received information bits.
In a typical receiver, accurate timing recovery is important for the performance of the demodulation unit (106) to be good. The impact on the link level performance of inaccuracies in the timing recovery loop depends on both the propagation environment and the architecture of the demodulation unit. The level of the performance degradation in case of non-ideal timing in the receiver is also heavily influenced by the sampling rate at which the receiver operates. It is well known that receivers operating at a rate higher than symbol rate, also referred to as over-sampled architectures, are less sensitive to timing errors than symbol-rate receivers (Digital Communications, John G. Proakis, 2nd edition, McGraw-Hill International). However, using over-sampling in the receiver almost invariably leads to a more complex solution both in terms of computations to be performed and memory requirements. Hence, in order to achieve an efficient implementation of the receiver it is desirable to avoid over-sampling the received signal.
High-Speed Downlink Packet Access (HSDPA) is an evolution of the Release 99 version of the 3GPP standard aimed at providing improved user experience through increased data rates and reduced end-to-end latency. These improvements are delivered through a combination of Incremental Redundancy (IR) and the use of higher-order modulation schemes. HSDPA extends the capabilities of 3GPP by introducing the use of the 16QAM modulation for the data bearing channels. It should however be noted that the 16QAM modulation is significantly more sensitive to impairments in the propagation medium, such as ISI, than the QPSK modulation used for the Release 99 version of the 3GPP system. In order to reduce the sensitivity of the receiver to channel impairments, more efficient, and more complicated, receiver architectures have been proposed. The Linear Minimum Mean Square Error (LMMSE) equaliser is an example of such an architecture (Chip-Level Channel Equalization in WCDMA Downlink, K. Hooli, M. Juntti, M. J. Heikkila, P. Komulainen, M. Latva-aho, J. Lilleberg, EURASIP Journal on Applied Signal Processing, August 2002). The LMMSE equaliser improves the performance of the demodulation unit by mitigating the distortions introduced by the propagation channel. The LMMSE equaliser can be implemented using a pre-filter Rake architecture (Equalization in WCDMA terminals, Kari Hooli, PhD thesis, 2003) where the conventional Rake receiver is preceded by a linear filter which aims at removing the ISI introduced by the channel. The link level performance of this receiver depends on the timing of the digital signal being processed. The performance also varies with the rate at which the received digital signal is sampled.
FIG. 2 presents the throughput performance of the pre-filter Rake receiver in fading propagation conditions for both cases of a symbol-rate implementation (one sample per chip) and of an over-sampled implementation (2 samples per chip). The throughput performance is presented versus the timing delay seen by the receiver. It can be seen that in the case of the symbol-rate receiver, the performance significantly varies with the timing used at the receiver. The difference in throughput between the best sampling point and the worst sampling point is approximately equal to 10%. This has to be contrasted against the performance of the over-sampled receiver where the throughput is essentially flat against the timing delay. Hence, using an over-sampled implementation of the receiver reduces/eliminates the need for accurate selection of the sampling point. However, the receiver operating at 2 samples per chip is computationally more complex than the symbol-rate receiver. The associated power consumption for the receiver will therefore be higher. The amount of memory required by the receiver is also larger. It is then important to note that the best throughput achieved by the symbol-rate receiver is in fact not worse than the best throughput achieved by the over-sampled receiver. This observation may seem surprising at first but can easily be understood by looking at the plot in FIG. 3 of the frequency spectrum of the digital signal. In the HSDPA system, the symbol rate is equal to 3.84 MHz. Hence, the symbol rate receiver will be able to process the signal in the frequency range −1.92 MHz to 1.92 MHz. It can be seen from FIG. 3 that most of the useful signal is confined in this range (the attenuation at 2 MHz is already equal to 10 dB). Hence, it is possible for a symbol-rate receiver to provide good performance provided that the ideal timing point can be identified in the receiver.